1. Field of the Invention
The present invention relates to a vehicle control unit for controlling a specific control target such as a braking device on a vehicle based on wheel speed. More particularly, the present invention relates to a control unit for controlling a specific control target adequately, even if the sensed wheel speed is not accurate because of limitations of a speed sensor, for instance.
2. Related Art
It is known that a vehicle control unit controls a specific control target such as a braking device on a vehicle on the basis of wheel speed sensed by a speed sensor. For example, an anti-skid control unit controls the braking force to each wheel on the basis of the wheel speed. The wheel speed is usually calculated from the frequency signal corresponding to the wheel rotation, which the speed sensor outputs. It is, however, difficult to calculate the accurate wheel speed when the wheel rotation is below a specific speed. This results from the fact that the frequency signal is not output correctly or is noisy, because the period of the frequency signal becomes long. Therefore, the vehicle control unit can make an undesired control when the wheel speed lowers below the specific speed.
To solve the above problem, it was proposed that the vehicle control unit controls braking only when the sensed wheel speed is more than a predetermined speed. When the sensed speed drops below the predetermined speed, the control unit stops or changes the control mode, determining that the sensed speed can not be used to control the specific control target adequately. However, there is also a possibility that the wheel speed can not be sensed adequately because of occurrence of fault in the speed sensor, even if the wheel speed is more than the predetermined speed. In this case, the vehicle control target can not be controlled adequately, because the vehicle control unit controls the vehicle control target on the basis of the incorrect wheel speed.